The present invention relates generally to fiber sensors, and more particularly to fiber sensors adapted for sensing load or pressure and strain or deformation changes in a host material.
Advancements in photonics and fiber optical technology have revolutionized high speed data communication. In the current state of the technology, the dominant role of optical fibers is for data transmission. Data in the form of photons or light is transmitted through optical fiber lines reliably and securely at very high speed and wide frequency band. To meet optimal requirements on integrity and quality of data transmission, the characteristics of the input and output light through the carrier optical fiber are desired to remain unchanged. Thus the fiber optical line is ideally intended to function as a conduit preserving the intensity, frequency, phase and polarity of the light beam as best possible. In addition to high speed and broadband, fiber optical lines remain immune to effects from electromagnetic interference, interception, heating and arching. These advantages over wire and wireless data transmission media inspired aggressive investment in fiber optical trunk lines in the past decades.
The next phase of the fiber optical revolution is modulation while in transmission. Characteristics of light beam are desired to alter in proportion to physical, thermal, chemical and biological changes. The intention to maximize modulation of a selected light beam property or properties is diametrically opposite to the desired objective of minimizing differences between input and output light in data transmission. Fiber segments that accentuate changes of characteristics desired to be preserved in data transmission are inserted in a network to detect or register relative changes. These fiber segments modulate transmitted light properties by virtue of induced changes in intrinsic or extrinsic conditions. Changes in intensity, frequency, phase, polarity and travel time have been exploited to develop a wide variety of sensors. The speed, reliability, accuracy, range and physical size make fiber optical sensors potentially attractive for a broad spectrum of applications. Incorporation of single or distributed fiber optical sensors within control loop circuits can lead to advances in smart materials and structures, intelligent transportation systems, energy conservation, clean environment, enhanced surveillance and in many fields of science and technology.
The main components of a control loop consist of transmission, detection, processing and actuation. Because of cost and relative development, practical circuits at present consist of mixed electronic and photonic components. The extent of photonic transmission would depend whether the circuit constitutes remote or local control and detection. For remote conditions, the bulk of the transmission can be in existing fiber optical lines. Local sensing and control loops can be fully electronic except for the sensing segment that would remain in photonic mode. Processing and actuating components would be in electronic mode for either local or remote loops. Circuits that entirely consist of photonic transmission, sensing, processing and actuation are not practical or economical at this time. The light input and output to and from the sensor can originate and terminate from a remote site or from close proximity to the sensor. The output is converted to electrical signal for processing and control decision. In turn, the processor controls actuators to initiate an adaptive response. This cycle continues to maintain set objectives for the system operation. An example would be to use sensor input and output to determine the axle load and wheelbase of a vehicle and the control decision may be to collect an appropriate toll charge for the vehicle. If successive sensors are linked, the toll can reflect also the travel time and speed of the vehicle. In addition, the toll can also reflect the time of day and level of traffic. Such a system would revolutionize the efficiency, energy conservation, law enforcement and safety of the highway system. While the technology for such a system exists, associated costs and complexity remain high.
Optical Fibers
An ordinary optical or plain fiber (FIG. 1) is a wave-guide through which light is propagated by continuous total internal reflection within the central core. The glass surrounding the core is known as the cladding. Both the core and cladding glass are dielectric materials. Because the index of refraction of the core is made higher than of the cladding, light waves remain trapped within the core in transmission. Depending on the wave modes utilized in transmission through the core, optical fibers may be classified as single mode or multi mode fiber. Multi mode fibers have core diameters commonly of either 50 or 62.5 μm and can be used for sensors that rely on intensity modulation. Whereas, single mode fibers have diameters in the range of 3 to 10 μm. Long distance communication and data transmission lines are generally of single mode fiber. Single mode and multi mode fibers cannot be coupled. Apart from other inherent limitations, multi mode fiber sensors cannot be integrated within existing communication fiber grid network. Commonly used fiber optical sensors for pressure and strain detection consist of segments of single mode sized fibers that contain Bragg gratings. When the sensor segment is subjected to changes in pressure and compliant strain, the grate spacing becomes altered. Such changes modulate the light wave passing through the sensor segment. Monochromatic light source of high intensity and interferometer detectors are usually required for the sensor system. The cost of such systems can be of the order of several thousand dollars.
It is a principal object and advantage of the present invention to provide an optical sensor that may be incorporated in a host material and used to sense load or pressure and strain or deformation changes in the host material.
It is another object and advantage of the present invention to provide an optical sensor based system for providing continuous feedback regarding the structural integrity of a structure composed of a host material in which the optical sensor is incorporated.
It is a further object and advantage of the present invention to provide an optical sensor that may be economically produced.
Other objects and advantages of the present invention will in part be obvious and in part appear hereinafter.